Common Protocols Several strategies have been proposed to solve the shared medium access problem. These strategies attempt, by various mechanisms, to strike a balance between achieving the highest- quality resource allocation decision and the overhead necessary to reach this decision. These strategies can be classified in three major categories: Fixed assignment Demand assignment Random assignment.
Fixed-Assignment Protocols In fixed-assignment strategies, each node is allocated a predetermined fixed amount of the channel resources. Each node uses its allocated resources exclusively without competing with other nodes. Typical protocols that belong in this category include Frequency-division multiple access(FDMA) Time-division multiple access (TDMA) Code-division multiple access(CDMA)
Demand Assignment Protocols The main objective of demand assignment protocols is to improve channel utilization by allocating the capacity of the channel to contending nodes in an optimum or near- optimum fashion. Unlike fixed-assignment schemes, where channel capacity is assigned exclusively to the network nodes in a predetermined fashion regardless of their current communication needs, demand assignment protocols ignore idle nodes and consider only nodes that are ready to transmit. Polling Reservation
Random Assignment Protocols In fixed-assignment schemes, each communicating node is assigned a frequency band in FDMA systems or a time slot in TDMA systems. This assignment is static, however, regardless of whether or not the node has data to transmit. These schemes may therefore be inefficient if the traffic sourceis bursty. In the absence of data to be transmitted, the node remains idle, thereby resulting in the allocated bandwidth to be wasted. Random assignment strategies attempt to address this shortcoming by eliminating preallocation of bandwidth to communicating nodes.
Random Assignment Protocols Random assignment strategies do not exercise any control to determine which communicating node can access the medium next. Furthermore, these strategies do not assign any predictable or scheduled time for any node to transmit. To deal with collisions,the protocol must include a mechanism to detect collisions and a scheme to schedule colliding packets for subsequent retransmissions. ALOHA CSMA CSMA/CD
Multiple Access Techniques
Frequency Division Multiple Access(FDMA) The FDMA scheme is used by radio systems to share the radio spectrum.Based on this scheme, the available bandwidth is divided into subchannels. Multiple channel access is then achieved by allocating communicating nodes with different carrier frequencies of the radio spectrum. The bandwidth of each node’s carrier is constrained within certain limits such that no interference, or overlap, occurs between different nodes. The scheme requires frequency synchronization among communicating nodes.
FDMA FDMA was the initial multiple-access Code technique for cellular systems Separates large band into smaller User 4 User 3 channels. User 1 User 2 Each channel has the ability to support user. Time Guard bands are used to separate y channel preventing co-channel nc interference ue eq Fr Narrow bandwidth (30 khz). f1
FDMA FDMA is a continuous transmission scheme as compare to TDMA because fewer bits are needed for synchronization and framing. In FDMA, as unique channels are assigned to each user, so FDMA systems have higher cell site system cost as compared to TDMA system. In FDMA, both the transmitter and receiver operates at the same time so FDMA Mobile units require duplexers. These also increase the cost of FDMA subscriber units and base station.
FDMA Advantages – Simple to implement in terms of hardware. – Fairly efficient with a small base population and with constant traffic. Disadvantages – Network and spectrum planning are intensive and time consuming. – Channels are dedicated for a single user, idle channels add spectrum inefficiency.
Time Division Multiple Access (TDMA In TDMA, a radio spectrum is divided into time slots. These time slots are allocated for each user to transmit and receive information. The number of time slots is called a frame. Information is transferred and received in form of frame. A frame is consists a preamble, an information message and trial bits. Preamble contains the address and synchronization information of both subscriber and Base Station to identify each other. Trial bits contain framing information.
Time Division Multiple Access (TDMA In TDMA, data transmission is not continuous and subscriber transmitter can be turned off which result in low battery consumption. In TDMA, handoff process is much simpler for a subscriber because of discontinuous transmission. In TDMA, duplexers are not required because different timeslots are used for transmission and reception. In TDMA, the rate of transmission is very high as compare to FDMA.
TDMA Entire bandwidth is available to the user for finite period of time. Code Users are allotted time slots for a 4 er channel allowing sharing of a single 3 er Us 2 r1 Us er channel. e Us Us Requires time synchronization. Time Each of the user takes turn in transmitting and receiving data in a cy en round robin fashion. u eq Fr
How it works? User presses Push-to-Talk (PTT) button A control channel registers the radio to the closest base station. The BS assigns an available pair of channels. Unlike FDMA, TDMA system also assigns an available time slot within the channel. Data transmission is not continuous rather sent and received in bursts. The bursts are reassembled and appear like continuous transmission.
TDMA Advantages & Disadvantages Advantages – Extended battery life and talk time – More efficient use of spectrum, compared to FDMA – Will accommodate more users in the same spectrum space than an FDMA system Disadvantages – Network and spectrum planning are intensive – Multipath interference affects call quality – Dropped calls are possible when users switch in and out of different cells. – Too few users result in idle channels (rural versus urban environment) – Higher costs due to greater equipment
TDMA Frame Structure Each sequence of 8 time slots is known as a TDMA frame
Code Division Multiple Access (CDMA) In CDMA, all users transmit information simultaneously by using the same carrier frequency. Each user has its own code word, which is orthogonal to other users. To detect the message, the receiver should know the codeword used by the transmitter.
Code Division Multiple Access (CDMA) CDMA includes the following features: In CDMA system many users share the same frequency. In CDMA unlike FDMA and TDMA the number of users is not limited. It has a soft capacity. But due to large number of users its performance degrades. In CDMA, each user operates independently with no knowledge of the other users.
CDMA CDMA is a spread Code spectrum technique used User 4 to increase spectrum User 4 efficiency. User 3 SS has been used in User 2 military applications due to User 1 Time anti-jamming and security. y nc ue eq Fr
Code-Division Multiple Access (CDMA) We start with a data signal with rate D, Which we call the bit data rate. We break each bit into k chips according to a fixed pattern that is specific to each user, called the users code.The new channel has a chip data rate of kD chips per second. Basic Principles of CDMA – D = rate of data signal – Break each bit into k chips Chips are a user-specific fixed pattern – Chip data rate of new channel = kD
CDMA Example If k=6 and code is a sequence of 1s and -1s – For a ‘1’ bit, A sends code as chip pattern <c1, c2, c3, c4, c5, c6> – For a ‘0’ bit, A sends complement of code <-c1, -c2, -c3, -c4, -c5, -c6> Receiver knows sender’s code and performs electronic decode function <d1, d2, d3, d4, d5, d6> = received chip pattern <c1, c2, c3, c4, c5, c6> = sender’s codeSu ( d ) = d1× c1 + d 2 × c 2 + d 3 × c3 + d 4 × c 4 + d 5 × c5 + d 6 × c6
CDMA Example User A code = <1, –1, –1, 1, –1, 1> – To send a 1 bit = <1, –1, –1, 1, –1, 1> – To send a 0 bit = <–1, 1, 1, –1, 1, –1> User B code = <1, 1, –1, – 1, 1, 1> – To send a 1 bit = <1, 1, –1, –1, 1, 1> Receiver receiving with A’s code – (A’s code) x (received chip pattern) User A ‘1’ bit: 6 -> 1 User A ‘0’ bit: -6 -> 0 User B ‘1’ bit: 0 ->6 unwanted signal ignored
CDMA Advantages – Greatest spectrum efficiency: – CDMA improves call quality by filtering out background noise, cross-talk, and interference – Simplified frequency planning - all users on a CDMA system use the same radio frequency spectrum. – Random Walsh codes enhance user privacy; a spread-spectrum advantage – Precise power control increases talk time and battery size for mobile phones Disadvantages – Backwards compatibility techniques are costly – Currently, base station equipment is expensive – Low traffic areas lead to inefficient use of spectrum and equipment resources
Random Access Random Access Methods – more efficient way of managing medium access for communicating short bursty messages in contrast to fixed-access schemes, each user gains access to medium only when needed -has some data to send drawback: users must compete to access the medium (‘random access’) collision of contending transmissions Random Access Methods in Wireless Networks – can be divided into two groups: ALOHA based-no coordination between users carrier-sense based-indirect coordination -users sense availability of medium before transmitting
Random Access Collision PeriodU ser 4U ser 3U ser 2 resched uledU ser 1 Tim e
ALOHA-based Random Access user accesses medium as soon as it has a packet ready to transmit – after transmission, user waits a length of time > round-trip delay in the network, for an ACK from the receiver – if no ACK arrives, user waits a random interval of time (to avoid repeated collision) and retransmits advantages: – simple, no synchronization among users required disadvantages: – low throughput under heavy load conditions – probability of collision increases as number of users increases max throughput = 18% of channel capacity
Slotted ALOHA time is divided into equal time slots –when a user has a packet to transmit, the packet is buffered and transmitted at the start of the next time slot – BS transmits a beacon signal for timing, all users must synchronize their clocks advantages: – partial packet collision avoided Disadvantages – throughput still quite low! – there is either no collision or a complete collision max throughput = 36% of channel capacity
Reservation ALOHA Time slots are divided into reservation and transmission slots / periods – during reservation period, stations can reserve future slots in transmission period – reservation slot size << transmission slot size – collisions occur only in reservation slots advantages: – higher throughput under heavy loads – max throughput up to 80% of channel capacity disadvantages: – more demanding on users as they have to obtain / keep ‘reservation list’ up-to-date R-Aloha is most commonly used in satellite systems satellite collects requests, complies ‘reservation list’ and finally sends the list back to users
Carrier Sense Multiple Access with CollisionDetect (CSMA/CD) With CSMA/CD, when an Ethernet device attempts to access the network to send data, the network interface on the workstation or server checks to see if the network is quiet. When the network is clear, the network interface knows that transmission can begin. If it does not sense a carrier, the interface waits a random amount of time before retrying. If the network is quiet and two devices try sending data at the same time, their signals collide. When this collision is detected, both devices back off and wait a random amount of time before retrying,
CSMA/CD Operation Carrier sense— Each computer on the LAN is always listening for traffic on the wire to determine when gaps between frame transmissions occur. Multiple access— Any computer can begin sending data whenever it detects that the network is quiet. (There is no traffic.) Collision detect— If two or more computers in the same CSMA/CD network collision domain begin sending at the same time, the bit streams from each sending computer interfere, or collide, with each other, making each transmission unreadable. If this collision occurs, each sending computer must be able to detect that a collision has occurred before it has finished sending its frame. Each computer must stop sending its traffic as soon as it has detected the collision and then wait some random length of time, called the back-off algorithm, before attempting to retransmit the frame.
Carrier Sense Multiple Access(CSMA) Disadvantages of ALOHA – users do not listen to the channel before (and while) transmitting – suitable for networks with long propagation delays Carrier Sense Multiple Access – polite version of ALOHA – Listen to the channel before transmitting if sensed channel busy, back-off (defer transmission), and sense channel again after a random amount of time if channel idle, transmit entire frame
Versions of CSMA Employs different node behaviour when channel found busy – non-persistent CSMA – persistent CSMA – 1-persistent CSMA – p-persistent CSMA
Persistence Methods What should a station do if the channel is busy? What should a station do if the channel is idle? 4 methods have been devised to answer these questions: the I-persistent method, the nonpersistent method, and the p-persistent method. Figure shows the behavior of three persistence methods when a station finds a channel busy. Persistent:- station sense the channel, if channel is ideal it transmits data. if there is already some traffic going on that it does not transmit the data. Keep sensing.
CSMA: Persistence Methods Behavior of 1-persistent, Nonpersistent, p-persistent method
Persistence Methods I-Persistent The I-persistent method is simple and straightforward. In this method,after the station finds the line idle, it sends its frame immediately (with probability I).This method has the highest chance of collision because two or more stations may find the line idle and send their frames immediately. Nonpersistent:- In the nonpersistent method, a station that has a frame to send senses the line. If the line is idle, it sends immediately. If the line is not idle, it waits a random amount of time and then senses the line again. The nonpersistent approach reduces the chance of collision because it is unlikely that two or more stations will wait the same amount of time and retry to send simultaneously. However, this method reduces the efficiency of the network because the medium remains idle when there may be stations with frames to send.
Persistence Methods p-Persistent :- The p-persistent method is used if the channel has time slots with a slot duration equal to or greater than the maximum propagation time. The p-persistent approach combines the advantages of the other two strategies. It reduces the chance of collision and improves efficiency. In this method, after the station finds the line idle it follows these steps: 1. With probability p, the station sends its frame. 2. With probability q = 1 - p, the station waits for the beginning of the next time slot and checks the line again. a. If the line is idle, it goes to step 1. b. If the line is busy, it acts as though a collision has occurred and uses the backoff procedure
Flow diagram for 1-persistent, Nonpersistent, p-persistent method
CSMA/CA (Collision Avoidance) Invented for wireless network where we cannot detect collisions Collision are avoided through the use of CSMA/CA’s three strategies: the interframe space, the contention windows, and acknowledgement IFS can also be used to define the priority of a station or a frame If the station finds the channel busy, it does not restart the timer of the contention window; it stops the timer and restarts it when the channel becomes idle
CSMA / Collision Avoidance Used where CSMA/CD cannot be used – e.g. in wireless medium collision cannot be easily detected as power of transmitting overwhelms receiving antenna – CSMA/CA is designed to reduce collision probability at points where collisions would most likely occur when medium has become idle after a busy state, as several users could have been waiting for medium to become available – key elements of CSMA/CA: IFS –interframe spacing –priority mechanism–the shorter the IFS the higher the priority for transmission CW intervals –contention window –intervals used for contention and transmission of packet frames Backoff counter–used only if two or more stations compete for transmission
CSMA/CA Algorithm Fram e to transm it Med ium No Wait until Id le? Trans end s Yes Wait IFS Wait IFS S till No If m ed ium becom es busy d uring the backoff S till No Id le? tim e, the backoff tim er is halted and Id le? Yes resum es when the m ed ium becom es id le. Yes Exp b/o while Transm it fram e Med ium id le Transm it fram e
Demand Assignment Protocols D e m a n d A s s ig n m e n t P r o t o c o ls Polling Reservation
Polling A widely used demand assignment scheme is polling. In this scheme, a master control device queries, in some predetermined order, each slave node about whether it has data to transmit. If the polled node has data to transmit, it informs the controller of its intention to transmit. In response, the controller allocates the channel to the ready node, which uses the full data rate to transmit its traffic. If the node being polled has no data to transmit, it declines the controller’s request. In response, the controller proceeds to query the next network node.
Polling The major drawback of polling is the substantial overhead caused by the large number of messages generated by the controller to query the communicating nodes. Furthermore, the efficiency of the polling scheme depends on the reliability of the controller. The main advantage of polling is that all nodes can receive equal access to the channel.
Reservation The basic idea in a reservation-based scheme is to set some time slots for carrying reservation messages. Since these messages are usually smaller than data packets, they are called minislots. When a station has data to send, it requests a data slot by sending a reservation message to the master in a reservation minislot. In a reservation-based scheme, if each station has its own reservation minislot,collision can be avoided. Moreover, if reservation requests have a priority field, the master can schedule urgent data before delay- insensitive data. Packet collisions can happen only when stations contend for the minislot, which use only a small fraction of the total bandwidth. Thus, the largest part of the bandwidth assigned to data packets is used efficiently.
Spread Spectrum Spread spectrum is designed to be used in wireless applications. The spread spectrum technique was developed initially for military and intelligence requirements. The essential idea is to spread the information signal over a wider bandwidth to make jamming and interception more difficult. In wireless applications, all stations use air (or a vacuum) as the medium for communication. Stations must be able to share this medium without interception and jamming.
Spread Spectrum To achieve these goals, spread spectrum techniques add redundancy; they spread the original spectrum needed for each station. If the required bandwidth for each station is B, spread spectrum expands it to Bss such that Bss » B. The expanded bandwidth allows the source to wrap its message in a protective envelope for a more secure transmission Eg:. expensive gift. We can insert the gift in a special box to prevent it from being damaged during transportation, and we can use a superior delivery service to guarantee the safety of the package.
Spread Spectrum Spread spectrum achieves its goals through two principles. 1. The bandwidth allocated to each station needs to be, by far, larger than what is needed. This allows redundancy. 2. The expanding of the original bandwidth B to the bandwidth Bss must be done by a process that is independent of the original signal. In other words, the spreading process occurs after the signal is created by the source.
Spread Spectrum After the signal is created by the source, the spreading process uses a spreading code and spreads the bandwidth. The spreading code is a series of numbers that look random, but are actually a pattern.
Spread Spectrum Input is fed into a channel encoder that produces an analog signal with a relatively narrow bandwidth around some center frequency. This signal is further modulated using a sequence of digits known as a spreading code or Spreading sequence.
Spread Spectrum Spread-spectrum techniques are methods by which a signal (e.g. an electrical, electromagnetic signal) generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, to prevent detection, and to limit
Spread Spectrum Types of spreading: – direct sequence spread spectrum (DSSS) – frequency hopping spread spectrum (FHSS)
Frequency hopping spread spectrum(FHSS) The frequency hopping spread spectrum (FHSS) technique uses M different carrier frequencies that are modulated by the source signal. At one moment, the signal modulates one carrier frequency; at the next moment, the signal modulates another carrier frequency. If an intruder tries to intercept the transmitted signal, she can only access a small piece of data because she does not know the spreading sequence to quickly adapt herself to the next hop.
Frequency Hoping Spread Spectrum(FHSS) Signal is broadcast over seemingly random series of radio frequencies – A number of channels allocated for the FH signal – Width of each channel corresponds to bandwidth of input signal Signal hops from frequency to frequency at fixed intervals – Transmitter operates in one channel at a time – Bits are transmitted using some encoding scheme – At each successive interval, a new carrier frequency is selected Channel sequence dictated by spreading code
Frequency Hoping Spread Spectrum Receiver, hopping between frequencies in synchronization with transmitter, picks up message Advantages – Attempts to jam signal on one frequency succeed only at knocking out a few bits
Frequency Hoping Spread Spectrum
Frequency Hopping Spread Spectrum (FHSS) FHSS
Frequency Hopping SpreadSpectrum (FHSS) Suppose we have decided to have eight hopping frequencies. This is extremely low for real applications and is just for illustration. In this case, Mis 8 and k is 3. The pseudorandom code generator will create eight different 3-bit patterns. These are mapped to eight different frequencies in the frequency table
Frequency Selection in FHSS
FHSS Performance Considerations Large number of frequencies used Results in a system that is quite resistant to jamming – Jammer must jam all frequencies – With fixed power, this reduces the jamming power in any one frequency band
Direct Sequence Spread Spectrum(DSSS The direct sequence spread spectrum technique also expands the bandwidth of the original signal, but the process is different. In DSSS, we replace each data bit with n bits using a spreading code. In other words, each bit is assigned a code of n bits, called chips, where the chip rate is n times that of the data
Direct Sequence Spread Spectrum(DSSS) Each bit in original signal is represented by multiple bits in the transmitted signal Spreading code spreads signal across a wider frequency band – Spread is in direct proportion to number of bits used One technique combines digital information stream with the spreading code bit stream using exclusive-OR
DSSS Direct Sequence Spread Spectrum (DSSS) Replace each data bit with n bits using a spreading code Each bit is assigned a code of n bits called chips